Although the present invention is not limited to the field of ion implanters, this field corresponds to a contemplated application and provides a useful context for understanding the invention. Hence, there follows a description of ion implanters.
Ion implanters are well known and generally conform to a common design as follows. An ion source produces a mixed beam of ions from a precursor gas or the like. Only ions of a particular species are usually required for implantation in a substrate, for example a particular dopant for implantation in a semiconductor wafer. The required ions are selected from the mixed ion beam using a mass-analysing magnet in association with a mass-resolving slit. Hence, an ion beam containing almost exclusively the required ion species emerges from the mass-resolving slit to be transported to a process chamber where the ion beam is incident on a substrate held in place in the ion beam path by a substrate holder.
Often, the cross-sectional profile of the ion beam is smaller than the substrate to be implanted. For example, the ion beam may be a ribbon beam smaller than the substrate in one axial direction or a spot beam smaller than the substrate in both axial directions. In order to ensure ion implantation across the whole of the substrate, the ion beam and substrate are moved relative to one another such that the ion beam scans the entire substrate surface. This may be achieved by (a) deflecting the ion beam to scan across the substrate that is held in a fixed position, (b) mechanically moving the substrate whilst keeping the ion beam path fixed or (c) a combination of deflecting the ion beam and moving the substrate. For a spot beam, relative motion is generally effected such that the ion beam traces a raster pattern on the substrate. The present invention relates to mechanical scanning of a substrate.
Our U.S. Pat. No. 6,956,223 describes an ion implanter of the general design described above. A substrate is held in a moveable substrate holder. While some steering of the ion beam is possible, the implanter is operated such that the ion beam follows a fixed path during implantation. Instead, the substrate holder is moved along two orthogonal axes to cause the ion beam to scan over the substrate following a raster pattern.
The movement of a substrate in a typical raster pattern is illustrated in FIG. 1. The substrate is moved continuously in a single direction (the fast-scan direction) to complete a first scan line. The substrate is then stepped down a short distance orthogonally (in the slow-scan direction), and a second line is then scanned. This combination of scan lines and stepwise movement results in scanning of the whole surface of the substrate through the ion beam. It is desirable to reduce the time taken for such scans to increase throughput of the ion implanter. However, increasing scan rates requires accurate and controlled acceleration, deceleration and translation of the substrate holder. Each acceleration and deceleration event of the substrate holder is generated by a motor or other moving force generator. As well as generating the desired substrate holder motion, an undesirable vibration in reaction to the movement occurs.
Referring again to FIG. 1, the illustrated raster pattern will first require the substrate and substrate holder to be accelerated in the horizontal (x) direction to a constant speed before the leading edge of the substrate reaches the ion beam. The substrate is then scanned through the beam at constant speed. Once the trailing edge is clear of the ion-beam, there follows a short deceleration period, and subsequently, or concurrently, the substrate holder is moved downwards (y direction) so that the reverse scan may be performed. Thus, there are acceleration and deceleration events in the horizontal direction, and similar events due to the stepwise movement in the vertical direction. Each of these events is likely to cause vibration.
US Patent Application published as US2004/0194565 describes a substrate scanner and semiconductor manufacturing apparatus. FIG. 2a shows schematically the substrate scanner disclosed in this document. The scanner comprises a tiered structure that has a substrate mounted on a carriage 120 that is, in turn, mounted on a reaction mass 100 that, in turn, is mounted on a base 140. To effect scanning, the carriage 120 (and hence substrate) is moved in the desired direction. At the same time, the reaction mass 100 moves in the opposite direction to provide a reaction force. In this document, the substrate carriage 120 is referred to as the “processing base”, the reaction mass 100 is referred to as the “moveable base” and the base 140 is referred to as the “fixed base”. While this arrangement reduces vibration, there remain a number of problems.
The substrate carriage 120 runs on guides on the reaction mass 100. To provide the necessary range of movement of the substrate relative to the ion beam, the reaction mass 100 must be longer than the amount of movement required by the substrate carriage 120, resulting in a bulky design.
Additionally, the reaction mass 100 moves in the opposite direction to the substrate carriage 120, as shown in FIGS. 2b and 2c. If the reaction mass 100 moves the same distance as the substrate carriage 120, then the reaction mass 100 will need to be as long as around twice the length of movement required by the substrate carriage 120, thereby adding to the bulk of the substrate scanner. Furthermore, if a similar implementation is used for each of the two orthogonal directions required to scan across a substrate, the consequent size of the substrate scanner will be further increased.
Mounting one movable device on top of another, such as mounting the movable substrate carriage 120 on top of the reaction mass 100, results in additional complexity in routing wiring and circuitry as well as requiring complex algorithms to generate the overall required movement of the substrate.
The present invention addresses the above described problems of the prior art and provides an improved substrate scanning apparatus.